Instructions

Process flour and salt in food processor until combined, about 5 seconds. Scatter butter over top and pulse until mixture resembles coarse sand and butter pieces are about size of small peas, about 10 pulses. Continue to pulse, adding ice water 1 tablespoon at a time, until dough begins to form small curds that hold together when pinched with fingers, about 10 pulses.
Transfer mixture to lightly floured counter and gather into rectangular-shaped pile. Starting at farthest end, use heel of hand to smear small amount of dough against counter. Continue to smear dough until all crumbs have been worked. Gather smeared crumbs together in another rectangular-shaped pile and repeat process.
Form dough into 6-inch disk, wrap tightly in plastic wrap, and refrigerate for at least 1 hour or up to 2 days. Let chilled dough sit on counter to soften slightly, about 10 minutes, before rolling. (Wrapped dough can be frozen for up to 1 month. If frozen, let dough thaw completely on counter before rolling.)
Roll dough into 12-inch circle between 2 large sheets of parchment paper (if dough sticks to parchment, dust lightly with flour). Slide dough, still between parchment, onto baking sheet and refrigerate until firm, about 15 minutes.
Adjust oven rack to middle position and heat oven to 375 degrees. Gently toss peaches, blackberries and Sucanat in bowl to combine. Remove top sheet parchment from dough. Mound peach mixture in center of dough, leaving 3-inch border around edge. Fold outer 3 inches of dough over filling, pleating dough every 2 to 3 inches as needed. Gently pinch pleated dough to secure, but do not press dough into fruit. Brush tart dough with beaten egg white.
Bake tart until golden brown and peaches are tender, about 1 hour, rotating sheet halfway through baking.
Let tart cool on baking sheet for 10 minutes. Using parchment, slide tart onto wire rack, then discard parchment. Let tart cool until juices have thickened, about 25 minutes. Serve warm or at room temperature.